Invoking scriptblocks with context

Invoke-WithContext.ps1

# InvokeWith context is useful when we have something like 
# ```
# $num = 1
# 1,2,3 | Assert-All -filter { $num -eq $_ }
# ```
# Where Assert-All is defined in a module where we invoke the filter script. The script needs to 
# stay bounded to the original scope to keep $name resolvable, but we also need to push the 
# $_ variable into the script block when invoking it. 


$m = New-Module -Name abc -ScriptBlock {
    function Invoke-WithContext {
        param(
            [Parameter(Mandatory = $true )]
            [ScriptBlock] $ScriptBlock, 
            [Parameter(Mandatory = $true)]
            [hashtable] $Variables)
        
        # this functions is a psv2 compatible version of 
        # ScriptBlock InvokeWithContext that is not available 
        # in that version of PowerShell

        # this is what the code below does
        # which in effect sets the context without detaching the
        # scriptblock from the original scope
        # & {
        #     # context 
        #     $a = 10 
        #     $b = 20
        #     # invoking our original scriptblock
        #     & $sb
        # }

        # a similar solution was $SessionState.PSVariable.Set('a', 10)
        # but that sets the variable for all "scopes" in the current 
        # scope so the value persist after the original has run which
        # is not correct, 

        $scriptBlockWithContext = {
            param($___context)

            foreach ($pair in $___context.Variables.GetEnumerator()) { 
                New-Variable -Name $pair.Key -Value $pair.Value
            }
            
            # this cleans up the variable from the session
            # the subexpression outputs the value of the variable
            # and then deletes the variable, so the value is still passed
            # but the variable no longer exists when the scriptblock executes
            & $($___context.ScriptBlock; Remove-Variable -Name '___context' -Scope Local)
        }

        $flags = [System.Reflection.BindingFlags]'Instance,NonPublic'
        $SessionState = $ScriptBlock.GetType().GetProperty("SessionState", $flags).GetValue($ScriptBlock, $null)
        $SessionStateInternal = $SessionState.GetType().GetProperty('Internal', $flags).GetValue($SessionState, $null)

        # attach the original session state to the wrapper scriptblock
        # making it invoke in the same scope as $ScriptBlock
        $scriptBlockWithContext.GetType().GetProperty('SessionStateInternal', $flags).SetValue($scriptBlockWithContext, $SessionStateInternal, $null)

        & $scriptBlockWithContext @{ ScriptBlock = $ScriptBlock; Variables = $Variables  }
    }
}




Get-Module abc, d | Remove-Module 
$m | Import-Module

$a = $null
$b = 11
$sb = { "-$a- -$b-" }        

# it is important that both the variables bound from the original scope
# and the variables in the context are correctly resolved when the scriptblock
# is invoked inside of the module above, this can only be achieved by not unbinding the 
# script from the original scope, so the middle line reads -10- -11- in the output. Also 
# the $a variable must not be persisted after the scriptblock has run so both the first and 
# last line should read "-- -11-"
"-$a- -$b-"
Invoke-WithContext $sb -Variables @{ A = 10 }
"-$a- -$b-"



$mm = New-Module -Name d -ScriptBlock { 
    $b = 100
    function Get-ScriptBlock { 
        { "-$a- -$b-" }    
    }
}
Get-Module d | Remove-Module 
$mm | Import-Module

$sb = Get-ScriptBlock
# here we get the scriptblock from the module
# and invoke it, the next line prints "-10- -100-" because 
# it is bound to the internals of the module, showing that we 
# don't rely on the caller scope as I did in my original solution
# $PSCmdlet.SessionState.PSVariable.Set('a', 10) wher the caller state
# would be modified, not the d module state
Invoke-WithContext $sb -Variables @{ A = 10 }